Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide, nitrogen oxides (“NOx”), sulfur oxides, and particulate matter. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Many different techniques have been applied to exhaust systems to clean the exhaust gas before it passes to atmosphere.
U.S. Pat. No. 6,805,849, for example, discloses an emission reduction method for reducing particulates, NOx, other pollutants. The method comprises passing the exhaust gas over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO2 and enhance the NO2 content of the gas stream, removing at least a portion of the particulates in a particulate trap, reacting trapped particulate with NO2, adding reductant fluid to the gas stream to form a gas mixture downstream of the trap, and passing the gas mixture over an SCR catalyst under NOx reduction conditions. The reductant fluid is suitably ammonia (NH3) but ammonia precursors including urea, ammonium carbamate can also be used.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new system that utilizes the counterflow injection of urea into the exhaust gas upstream of an SCR catalyst.